Fetch attachments from approved GCI task instances

For GCI task which did not have a PR assessment item, the most recent attachment on approved GCI task instances are likely to be the outputs of the task, and should be fetched into the private folder.

The fetcher should not push these private artifacts to the public folder to be published. This needs to be manually done to ensure those artifacts had correct licensing and were indeed the correct output of the task.

A naming convention needs to be established so that those attachments moved to the public folder by an org admin will be linked to the task, and probably presented as a gallery.

A coala-specific implementation detail, maybe best dealt with as a separate enhancement, is some of the coala tasks required uploading the artifacts to Behance or DeviantArt, and those links need to be extracted from the task instance instead of attachments.